Axial piston pumps and motors as used in the aerospace industry operate under harsh environmental conditions and often are subject to significant stress concentration levels. That stress arises because pistons reciprocate at high velocities and simultaneously rotate in relation to piston bores formed within a cylinder block. In certain applications, such as an aircraft integrated drive generator (IDG), the relationship of each piston to its respective piston bore within the cylinder block is preferably controlled within a tight diametral clearance range, such as 0.0001" to 0.0004". In such an arrangement to produce 400 Hz electric power, for example, oil is pumped into and/or out of the piston bores at up to approximately 6000 psi.
Each cylinder block of an IDG typically contains nine pistons; and each IDG typically contains four cylinder blocks per aircraft engine. Thus, to increase service reliability of the aircraft and reduce periodic maintenance, minimal wear of the pistons on associated cylinder block bores is desirable. In order to protect the piston bores from the harsh, aircraft operating environments, a bushing may be inserted into each of the piston bores to reduce sliding friction and wear caused by piston movement. An example of a preferable piston embodiment for movement within a cylinder block is disclosed in U.S. Pat. No. 3,319,575 to Havens.
One previous method of manufacturing the cylinder block with bushings includes diffusion bonding each of the nine bushings to a respective piston bore surface. This process requires separate machining of each of the piston bores within a tightly controlled tolerance measurement to match outer diametral bushing dimensions. In addition, separate manufacture of each of the nine bushings within correspondingly tight inner and outer diametral dimensions is required. Such a process further requires interference fitting each of the bushings into the piston bores, before applying pressure to diffusion bond the material of the bushing, usually bronze, and the material of the cylinder block which defines the surface of the piston bore, usually steel, together.
In addition, each cylinder block may include a valve plate, sometimes referred to in the art as a "port plate", which is secured to an end of the cylinder block. The valve plate rotates with the cylinder block in operation to regulate the amount of propulsive oil entering and exiting each piston bore. Such a plate is preferably constructed from the same wear resistant material as the bushings. The aforementioned diffusion bonding process has also been utilized to secure the valve plate to the end of the cylinder block. However, this requires an additional piece of hardware to be manufactured and separately bonded to the cylinder block.
Thus, the prior diffusion bonding process poses significant manufacturing obstacles, as each machining operation and processing step requires additional labor and production cost; and the tightly toleranced dimensions often result in an increased amount of scrap material. These obstacles address the downfalls associated with the construction process. However, more serious performance-driven problems can be encountered by using a diffusion bond to secure the bronze bushings and the valve plate to the cylinder block. During a diffusion bonding process, the bronze material of the bushings and the valve plate never reach a molten state. Because of this, gas and other impurities can become entrapped within the bronze material. Once the pressure applied during the process is relieved, the bushing may be left with significant amounts of porosity. In addition, diffusion bonding can result in microshrinkage, as volumetric changes in the bronze material occur during cooling. Microshrinkage results in microscopic voids near the surface of the bushing and the valve plate surfaces. These entrapped gases and voids in the bushing surfaces create stress risers. Since bronze material which has been diffusion bonded tends to be soft (i.e. 18-45 HR.sub.B), the voids and/or entrapped gases greatly decrease the bushings' ability to reduce friction and wear caused by piston movement. Thus, the piston in operation creates a wear path in the bushing. The extra clearance in the piston bore caused by this piston wear can trigger fluid leakage in the cylinder block and piston assembly. Excess leakage can result in low charge pressure, and electrical performance frequency ratings of the IDG from being reached. Eventually, the wear can cause the hydraulic unit to completely malfunction.
Accordingly, objects of the present invention include providing an improved process of forming a copper-based alloy casting substantially free of voids or entrained gas on a cylinder block, and providing an advanced cylinder block design, which has advantageous wear reduction characteristics. Other objects of the invention include the following:
(i) to provide a cost-effective method for manufacturing a cylinder block with a copper-based alloy casting; PA1 (ii) to eliminate separate construction operations for each bushing and the valve plate; PA1 (iii) to eliminate separate assembly operations for each bushing within the cylinder block; PA1 (iv) to eliminate separate assembly of the valve plate to the cylinder block; PA1 (v) to create a copper-based alloy casting with superior wear characteristics including ductility, strength, hardness, and cavitation resistance; PA1 (vi) to effectively reduce microshrinkage of the bushing and valve plate surfaces during processing; PA1 (vii) to effectively control porosity to a specified location of the copper-based alloy during the manufacturing process; PA1 (viii) to effectively isolate impurities of the casting material to a specified location of the copper-based alloy during the manufacturing process; PA1 (ix) to increase service reliability of the cylinder block; and PA1 (x) to provide a plurality of wear-resistant annular members which are functionally equivalent to the bushings and valve plate, but integrally cast as part of the cylinder block.